This application is based on Patent Application No. 10-216022 (1998) filed Jul. 30, 1998 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a printing apparatus and a printing method for printing an image on a printing medium by ink dots, which are formed as a result of discharging ink. More particularly, the present invention relates to a printing apparatus and a printing method for printing a color image by using more than one inks in different concentrations for black.
2. Description of the Prior Art
In recent years, copying machines, information-processing apparatuses (e.g., word processor and computer), and communication apparatuses become widely available. As one of their image-forming and printing means, an apparatus using an inkjet printing head for printing an image in digital form has been sprung into wide use. In addition, the above information-processing apparatuses and communication apparatuses become possible to provide high-quality and colored visual information. Therefore, the request of a printing apparatus with the capability of providing a high-quality color image has been increased.
For the sake of providing a high-quality color image, it is well-known that a printing apparatus is configured so as to use a printing head which is prepared by accumulating and arranging a plurality of printing elements to correspond to minute pixels (hereinafter, referred as a multi-head). In an inkjet printing system, for example, it is well-known that a printing head having high-density orifices for ejecting ink and high-density liquid passages is used. For a color image printing, a plurality of multi-heads may be included so as to correspond to cyan, magenta, yellow, and black inks, respectively.
However, the present technology has a certain limit to accumulate orifices and liquid passages in high density, so that there is also a certain limit to make a minute pixel. In this case, each dot that forms the corresponding pixel becomes one in a certain size that depends on such a limitation, resulting in troubles of image qualities. Such troubles include granularity in a lower dense portion such as a high-light portion of an image.
Instead of increasing the accumulation densities of orifices and liquid passages (i.e., instead of decreasing the size of one pixel), the so-called multi-drop method is well known in the art. The method lessens the discharged volume of ink by decreasing a diameter of each orifice or the like and forms a dot by discharging several ink droplets on the same pixel in accordance with an image to be printed. The method allows a comparatively small diameter of a dot to be printed on a printing medium, so that the granularity on the lower dense portion such as a high-light portion of an image can be improved. If the ink droplet to be discharged is too small, there is a tendency of causing unstable ink discharge. Thus, there is a certain limitation of decreasing the size of an ink droplet. As a result, there is also a certain limitation of increasing an image quality. In this method, furthermore, the number of ink droplets to be discharged on one pixel increases as the concentration increases. Thus, a printing operation takes much time and results in drop in a printing speed. Consequently, a high-quality image formation and an increase in printing speed are mutually contradictory.
A gray-scale printing method is known as another technique of forming a high-quality image without increasing the accumulation density of orifices. The gray-scale printing method uses different concentrated inks of the same color. In this method, a high-light portion or the like is printed using a comparatively low concentrated ink (hereinafter, referred as a light ink) so that the granularity does not attract attention, while high-concentrated portion is printed using a comparatively high concentrated ink (hereinafter, referred as a dark ink). Unlike the multi-drop system, therefore, it becomes possible to print the high-concentrated portion without increasing the number of ink droplets to be discharged on one pixel (i.e., the number of ink discharges). Consequently, the gray-scale printing method is able to avoid a decrease in printing speed.
The above method using light and dark inks may be improperly applied to some cases, such as medical images, especially a monochrome image such as an X-ray image. The medical image is printed on a transparent printing medium such as a transparent film. Even though such a medical image is provided as a monochrome one, it requires a comparatively precise gradation. It is noted that an absolute volume of light passing through an image on a transparent printing medium is greater than an absolute volume of light reflecting from an image on a reflective one. Accordingly, a visually recognizable dynamic range of the transparent image is more extended than that of the reflective image, so that the number of recognizable gray-scale levels of the former is greater than that of the latter. Therefore, a visual resolution with respect to an ink concentration is increased. As the difference in concentration of each pixel is also recognized in a case where light and dark inks are used, the image may be recognized as a rough one. For improving this fact, there is an idea of increasing the number of gray-scale levels for each pixel. However, the above method requires increasing numbers of different types of light and dark inks with their corresponding multi-heads, resulting in the high cost.
The present inventors and their colleagues make a suggestion concerning another method in Japanese Patent Application No.9-323435 (1997). The method includes the step of printing different concentrated inks of the same color (especially different concentrated black inks) on the same pixel. In this case, for example, four drops of such inks are printed one by one on the same pixel to provide gray-scale levels more than the number of levels per one pixel in the prior method. It means that more gray-scale levels can be represented without substantially increasing the number of light and dark inks and the number of multi-heads.
The above printing procedure uses a pseudo-halftones processing method by means of a binary or multi-level processing, such as a dither method, an error diffusion method, and a mean concentration maintenance method.
The dither method is of representing each pixel data in a binary form that depends on a threshold level of each pixel determined by a dither matrix.
The error diffusion method represents an image data of a target pixel in a binary form (converting into the darkest level or the lightest level and distributes a difference (i.e., an error) between the binary level and the pre-binary level on the peripheral pixels to perform an addition of levels. See, for example, R. FLOYD and STEINBERG, xe2x80x9cAN ADAPTIVE ALGORITHM FOR SPETIAL GRAY SCALExe2x80x9d, SID 75 DIGEST pp. 36-37.
The mean concentration maintenance method is described in Japanese Patent Application Laying-open No. 2-210962 (1990) and so on. In this method, a binary form depending on a threshold level represents the target data. The threshold level is determined by using binary data of a neighborhood of the target pixel which is previously represented in a binary form or using binary data obtained by representing the target pixel by one of two levels, black or white.
In addition, a multi-level processing may be allowed by revising various kinds of binary methods.
In the case of the above method for overlaying different concentrated inks, there are following problems.
A printing medium has a threshold volume of ink to be accepted. If the volume of ink to be discharged on the same pixel exceeds such a threshold, excess ink cannot be absorbed into the printing medium and results in xe2x80x9cink overflowxe2x80x9d. In this case, furthermore, a printing medium may be wrinkled (i.e., xe2x80x9ccocklingxe2x80x9d) by an effect of the absorbed ink.
Take, for example, an output of a medical image such as a color echogram. It requires both a gradation of black and an output in multiple colors, if light and dark inks of black are discharged at almost the acceptable volume of ink absorption, there are no remaining abilities to absorb additional inks of cyan, magenta, yellow. Thus, the problems described above may be caused when those inks are further discharged on the same pixel.
For placing emphasis on an output in multiple colors, there is another problem in which a sufficient gradation of black cannot be attained. The reason is that a degree of freedom for overlaying light and dark inks of black and its derived colors is limited to allow a sufficient absorption of any of cyan, magenta, and yellow color inks.
It is an object of the present invention is to provide a printing apparatus and a printing method that avoid any problems including an ink overflow to allow a color image formation with a sufficient gradation of black.
In the first aspect of the present invention, there is provided a printing apparatus that forms an image with use of a plurality type of black ink having different concentrations and other color inks, comprising:
region-determining means for determining whether a color represented by image data belongs to an achromatic region or a chromatic region on the basis of the image data; and
gray-scale combination means for determining one of the type of ink and a volume of ink to be applied to one pixel on the basis of the image data for each region determined by the region-determining means.
In the second aspect of the present invention, there is provided a printing method using a printing apparatus that forms an image with use of a plurality type of black ink having different concentrations and other color inks, comprising the steps of:
region-determination for determining whether a color that represented by image data belongs to an achromatic region or a chromatic region on the basis of the image data; and
gray-scale combination for determining one of the type of ink and a volume of ink to be applied to one pixel on the basis of the image data for each region determined by the region-determination step.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.